The fundamental performance or reliability criterion is the acceptable failure rate. This criterion is based on the consequence of failure and on the expected life of the equipment. Therefore, the failure rate of transmission lines and substation equipment may be different.

Transmission Lines
The performance/reliability criterion for lightning is normally specified as the number of flashovers per 100 km-years. For switching surges, the flashover rate is normally specified in terms of flashovers per number of switching operations.

However, the highest magnitude switching surges normally occur when reclosing, which is normally caused by a fault associated with lightning. Thus, the two separate criteria may not be appropriate in specifying the line reliability.

Another criterion, denoted as the storm outage rate, is the number of unsuccessful reclosures per year and is obtained by multiplying the lightning flashover rate for the line in units of flashovers per year by the switching surge flashover rate in terms of flashovers per switching operation.

For example, assuming the lightning flashover rate to be two per year and the switching surge flashover rate to be one per 100 switching operations, the storm outage rate is two per 100 years assuming one reclosing operation per year. Both the storm outage rate and the lightning flashover rate may be important since the lightning fault creates voltages “dips” or depressions in power frequency voltage, which may affect customer power quality.

For transmission lines, lightning flashover rates vary with system voltage and may range from 0.5 for EHV systems to 20 per 100 km-year for HV systems. Although lines are being designed for switching surge flashover rates between 1 and 10 flashovers per 100 switching operations, due to other conservative assumptions, switching surge flashovers are relatively rare.

The commonly used reliability criterion for lightning is the MTBF.

Because the consequence of failure within the station is greater than the consequence of a flashover of a single line, the station reliability criterion is higher than the line by a factor of 10. In addition, transformers, other wound devices, and equipment with non self-restoring insulation may be arrester protected due to the consequence of failure.

Different criteria are usually applied to air- and gas-insulated stations. For example, the MTBF for air-insulated stations varies between 50 and 200 years, whereas for the gas-insulated station, the MTBF has been set as high as 800 years.

The basis for the increase is the consequence of failure in the gas-insulated station that may require significant outage and repair times.

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